Rates of ischemic stroke during warfarin treatment for atrial fibrillation

Background and Purpose — Recent evidence suggests there may be an increased risk of ischemic stroke immediately following warfarin initiation. The researchers examined the rate of ischemic stroke among patients with atrial fibrillation newly started on warfarin therapy.

Methods — The researchers conducted a population-based cohort study among Ontario residents age 66 years and older with atrial fibrillation who received warfarin between April 1st, 1997 and March 31st, 2010. Each patient was followed for up to 5-years in 30-day intervals. For each interval the researchers determined the rate of ischemic stroke.

Results — After 5 years, the cumulative incidence of ischemic stroke among new users of warfarin (n=148,446) was 4.0% (n=6,006). The risk was highest during the first 30 days following initiation (6.0% per person-year (PPY), 95% confidence interval (CI) 5.5%-6.4%) compared to the remainder of follow-up (1.6% PPY, 95% CI 1.5%-1.6%), and increased with higher baseline CHADS₂ scores. Less frequent monitoring may have contributed.

Conclusion — In a large cohort of older patients with atrial fibrillation, the researchers observed the highest rate of ischemic stroke in the first 30 days following warfarin initiation. While causation cannot be established given the observational nature of this study, our findings highlight the need for future research in this population.